The names of unnicknamed Pokémon will be rechecked upon transfer, leading into foreign Pokémon to revert to the species name in their origin language if they had obtained a name of another origin by evolution and Pokémon from before Generation V to have their all-uppercase names updated to be capitalized normally. If a Pokémon's nickname contains a word which is prohibited in the Generation VI games, the Pokémon's nickname will be erased, reverting to the default species name.
After researching further it may be because Avalanche has the following words in it's name:
Ava- Ava is an Italian word, it basically means "to f***" in English.
Ala- Ala is a French word used to describe a male individual, the word in English means "b****d" or "backstabber".